Serious thanks to anyone who attempts to help me with this.So, I was trying to come up with the isomers of C6H12, simply because I'm trying to get better at finding them (practice makes perfect!) but after I came up with what I thought was all of them, a quick internet search suggests that there are only 18. I've got 19. I'm not sure where I've gone wrong - maybe I've accidentally written the same one twice, in the sense that a methyl group got attached to an end carbon in my hurry. Anyway, here's what I have:1. Cyclohexane2. Methylcyclopentane3. 1,1-dimethylcyclobutane4. 1,2-dimethylcyclobutane5. 1,3-dimethylcyclobutane6. 1-hexene7. 2-hexene8. 3-hexene9. 2-methyl-1-pentene10. 3-methyl-1-pentene11. 4-methyl-1-pentene12. 2-methyl-2-pentene13. 3-methyl-2-pentene14. 4-methyl-2-pentene15. 3,3-dimethyl-1-butene16. 2,3-dimethyl-1-butene17. 2,3-dimethyl-2-butene18. 1,2,3-trimethylcyclopropane19. 1,1,2-trimethylcyclopropane

Thankyou, thankyou, thankyou if you take the time to read through these to point out what will probably be an obvious mistake We haven't done cis or trans yet, I don't know if that affects it in any way, so I'm hoping that I haven't butchered IUPAC convention too badly (Also, no, this wasn't homework - it was genuine interest/me wanting to be able to draw isomers properly)

I thought the same thing at first, but then I realized they were not. 1,2-dimethylcyclobutane has the two methyl groups on adjacent carbons (i.e. same side of the square), while 1,3- has the two methyl groups on opposite carbons (i.e. diagonally across the square).

I have read through the list and nothing jumped out at me as a repetition.

When you do things right, people won't be sure you've done anything at all. - Futurama 4x08 Godfellas

I thought the same thing at first, but then I realized they were not. 1,2-dimethylcyclobutane has the two methyl groups on adjacent carbons (i.e. same side of the square), while 1,3- has the two methyl groups on opposite carbons (i.e. diagonally across the square).

Oh, you're right. For some reason I mentally pictured 1,1 as groups on adjacent carbons rather than on the same one, and then was one off when I pictured the next two. Strangely enough I didn't make that mistake going through the rest of the list.

I agree there are no repeats on the list. If you allow for stereo-isomers there will be many more, but other than that I think this list is complete.

I thought the same thing at first, but then I realized they were not. 1,2-dimethylcyclobutane has the two methyl groups on adjacent carbons (i.e. same side of the square), while 1,3- has the two methyl groups on opposite carbons (i.e. diagonally across the square).

Oh, you're right. For some reason I mentally pictured 1,1 as groups on adjacent carbons rather than on the same one, and then was one off when I pictured the next two. Strangely enough I didn't make that mistake going through the rest of the list.

I agree there are no repeats on the list. If you allow for stereo-isomers there will be many more, but other than that I think this list is complete.

Theres a trans and a cis version of both. And the 1,2 cis has two stereoisomers.

Then there is ethylcyclobutane, proylcyclopropane, 1-methyl-1-ethylcyclopropane, cis-1-methyl-2-ethylcyclopropane, ditto trans (R and S), the trimethyl variants and so on.

2-methyl-2-pentene and 4-methyl-2-pentene are duplicates. The reason for this is that since the double bond will have the same numbers no matter how you number the carbons in the main carbon chain, the numbering is done so as to give the lowest possible number to the greatest possible number of substituents. Therefore, if you have the methyl group at the "4" position, you start numbering at that side of the chain so the 4 position is now the 2 position. I hope that makes sense. Just draw the 2 isomers out if it doesn't and you'll see that they're superimposable on each other.

Foomaster wrote:2-methyl-2-pentene and 4-methyl-2-pentene are duplicates. The reason for this is that since the double bond will have the same numbers no matter how you number the carbons in the main carbon chain,

2-Pentene is not symmetric - there are two carbon atoms to one side of the double bond, and 3 carbon atoms to the other side. Reversing the atom numbering would make it 3-pentene.

Just to point out, cis/trans isomerism will affect many of your alkenes (except the ones with duplicated groups on either side). There will also be cis/trans isomerism in your ring compounds - there will be two diastereoisomers of 1,2-dimethylcyclobutane, for example, with the methyl groups on the same side of the ring (cis) and opposite sides (trans). Make a model if that isn't obvious. There will also stereoisomers (chirality) but that's a bit more complicated.And I believe IUPAC favours pent-1-ene, rather than 1-pentene. Not that anybody uses standard names that much anyway.

Funnily enough, we did geometric isomers two days after I posted... that serves me right for being impatient Yes, I noticed the pent-1-ene sort of notation in our textbook but for some reason all of our teachers are teaching it to us with the number out the front. But thankyou, I might follow up on the cis/trans isomers in the cyclics (once exams are over and I have enough time :S).

So.. I'm not sure whether I'm too late with this post (and sorry about the bad english... I'm Icelandic) but I specifically registered to this forum because of this thread I'm currently studying Chemistry in the local university so I thought it would be a fun task to answer some of this stuff before doing... well anything I'm supposed to be doing.

Regarding those isomers of yours... I'm pretty sure theyre more than just 19. I did not take the time to draw them but instead I used this awesome database (which I normally do 'cause I'm that lazy) and found well about 40 isomers. And thats just what the database says, there could be more or not... I don't really understand the database fully yet. But here is the link anyway http://webbook.nist.gov/chemistry/form-ser.html.en-us.en and for your Isomer you get this link http://webbook.nist.gov/cgi/cbook.cgi?Formula=C6H12&NoIon=on&Units=SI.Now you just ignore the polymerized - not really in the definition of an isomer- and deutrated chemicals (the ones with D instead of H which are isotopes not isomers) and then you have your isomers for C6H12.

AFedchuck wrote:And I believe IUPAC favours pent-1-ene, rather than 1-pentene. Not that anybody uses standard names that much anyway.

Yeah here in NZ we're taught this from the start and get penalised for not using IUPAC conventions, but then when I read on the internet they use old names like formaldehyde instead of methanal, etc. Why is this? Doesn't this defeat the purpose of an international convention?

My teacher tells me that a) chemists (/scientists in general) are lazy and use shorthand wherever possible, and b) they've been in use for so long that it's hard to get people to change.Another example: acetic acid (or acetates) vs ethanoic acid (or ethanoates).

Phoenix112358 wrote:Yeah here in NZ we're taught this from the start and get penalised for not using IUPAC conventions, but then when I read on the internet they use old names like formaldehyde instead of methanal, etc. Why is this? Doesn't this defeat the purpose of an international convention?

Sometimes its easier to say things like benzene, instead of cyclohex-1,3,5-triene (i think that's right... i'm a bit rusty on my naming conventions, even tho i only took orgo 2 semesters ago ><) Helps to keep the names relatively short.

Phoenix112358 wrote:Yeah here in NZ we're taught this from the start and get penalised for not using IUPAC conventions, but then when I read on the internet they use old names like formaldehyde instead of methanal, etc. Why is this? Doesn't this defeat the purpose of an international convention?

Sometimes its easier to say things like benzene, instead of cyclohex-1,3,5-triene (i think that's right... i'm a bit rusty on my naming conventions, even tho i only took orgo 2 semesters ago ><) Helps to keep the names relatively short.

Especially since cyclohex-1,3,5-triene is an inaccurate name for benzene (the whole "it is not really alternating single and double bonds" thing.)

More generally: Systematic names are nice in that they are unambiguous, but they rapidly gets ridiculously complicated, so people are going to keep simplifying them. Try giving the correct IUPAC name for even a short peptide. At the other end of the scale there are the simple familiar compounds that are hard to change. People are going to continue saying acetic acid rather than ethanoic acid for a long time, and people are never going to say hydrogen oxide rather than water.

AFedchuck wrote:And I believe IUPAC favours pent-1-ene, rather than 1-pentene. Not that anybody uses standard names that much anyway.

The IUPAC form is more sensible, chemically speaking, but it sounds and feels awkward when spoken, compared to the form with the leading number. And numbers inserted into words aren't as linguistically natural as numbers in prefix position. How's a chemist supposed to sound cool rattling off impressive chemical names when they risk stumbling over these stupid internal numbers?

Meteorswarm wrote:Not *all* of those are isomers; they list 3-pentene, (Z) 3-pentene and (E) 3-pentene as separate entities; 3-pentene is only meaningful if you've somehow got enough heat to rotate that bond around, making the E-Z distinction useless, and then the (E) and (Z) isomers are meaningless, since they don't really exist as independent entities.

I fail to understand this statement. (Z) 3-pentene and (E) 3-pentene ARE separate entities and are two different isomers and are defined as such (http://en.wikipedia.org/wiki/E-Z_notation). Sure (E) 3-pentene is more stable because its not as sterically hindered as (Z) 3-pentene that doesnt mean they dont exist as independent entities. They have different heat of evaporation values and different this and that. Plus if you raise the heat you won't ever get a rotation about a double bond, you actually have to break the bond in order to rotate it and then form it again (that could very well happen at high temperatures but it's not rotation).

Put simply, E-Z notation is just and extension of the Cis/Trans isomer notation and is always meaningfull.

And for the IUPAC discussion... I've always just thought of that naming form as a thing to put on papers and reports (homework and exams too) only written once though and then I use the simplified names, but when I'm at the lab I'm certainly not going to say propanone instead of acetone. Maybe chemists are just thick headed and stick tightly to the old convention because thats the only one they know.

Meteorswarm wrote:Not *all* of those are isomers; they list 3-pentene, (Z) 3-pentene and (E) 3-pentene as separate entities; 3-pentene is only meaningful if you've somehow got enough heat to rotate that bond around, making the E-Z distinction useless, and then the (E) and (Z) isomers are meaningless, since they don't really exist as independent entities.

I fail to understand this statement. (Z) 3-pentene and (E) 3-pentene ARE separate entities and are two different isomers and are defined as such (http://en.wikipedia.org/wiki/E-Z_notation). Sure (E) 3-pentene is more stable because its not as sterically hindered as (Z) 3-pentene that doesnt mean they dont exist as independent entities. They have different heat of evaporation values and different this and that. Plus if you raise the heat you won't ever get a rotation about a double bond, you actually have to break the bond in order to rotate it and then form it again (that could very well happen at high temperatures but it's not rotation).

You did fail to understand it - the point was as far as I can tell that that 3-pentene is not a meaningful name. Also, the double bond could be considered just an energy barrier - just like the barrier between the various staggered conformers of ethane, or to take an intermediate example, the CN bond in DMF. If you heat it enough, the molecule will change between the various conformations relatively freely - I'd call that rotation. However, whether the molecule will be stable at such an elevated T is not certain. Energy barriers in ethene are ~250kJ/mol (back of the envelope if kT = barrier, T=30000K - this sounds too high to me).

I´m counting 24 isomers. If we´re talking about constitutional isomers, of course. And yes, I also registered for this thread I think the additional one is "Ethyl-cyclobutane"edit: Found another one: isoPropyl-cyclopropane. I hope that´s it

I´m counting 24 isomers. If we´re talking about constitutional isomers, of course. And yes, I also registered for this thread I think the additional one is "Ethyl-cyclobutane"edit: Found another one: isoPropyl-cyclopropane. I hope that´s it

Interesting. We haven't done all the nomenclature in the entire IUPAC system, so would you be able to explain what the 'iso' bit means, in terms of the structure? From some quick googling it seems like in this case it's two carbons branched off from one (I'm terrible at explaining things at this point) but I'd prefer a more precise definition (I know cis/trans geometric isomerism, that's all.)And thankyou!

Edit: In regards to the 3-pentene discussion above... am I right in thinking that 3-pentene is, well, 2-pentene? Also,

These would be the ones which can exhibit geometric (as in cis/trans) isomerism, correct? That grouping you did was really helpful, Hueschel. (Methyl-pentane in the cycloalkanes should be methylcyclopentane, yeah?)

(Methyl-pentane in the cycloalkanes should be methylcyclopentane, yeah?)

Yes, sorry for that.

I'm ticked that I missed the ethylcyclobutane and the isopropylcyclobutane (2-propylcyclopropane?)

2-propylcyclopropane and isopropylcyclopropane are not the same. Actually "2-propylcyclopropane" doesn´t exist, because you´d have to say "1-propyl-cyclopropane" or simply "propylcyclopropane" and that´s just a normal propylgroup. If you want to use a 2 to express an isopropylgroup, you need to declare the isopropylgroup as main hydrocarbon chain. For example "2-Hydroxypropane" is the same as "isopropylalcohol".

I figured 2-propylcyclopropane would be clear enough that the propyl is connected at the #2 carbon to the cyclopropane preciseley because there is no #2 carbon on the cyclopropane in this system. It could be made clearer by declaring it (2-propyl)cyclopropane or even 1-(1-methylethyl) cyclopropane. I was looking through my (very old) CRC and they use the isopropyl terminology. Maybe it's common enough to use? I dunno.